Battery protection system and process for charging a battery

ABSTRACT

A battery protection system (20) controls a process for charging a battery pack (15). A hysteresis comparator (54) senses a charging current flowing through the battery pack (15) and switches off a charging switch (31) to interrupt the charging current when the charging current reaches an upper limit. A transient current is then generated by an inductor (34). The hysteresis comparator (54) senses the transient current flowing through the battery pack (15) and switches on the charging switch (31) to regenerate the charging current when the transient current decreases substantially to zero. Periodically, a battery monitoring circuit (40) switches off the charging switch (31) and measures an open circuit voltage across each battery cell in the battery pack (15). In response to the open circuit voltage of a battery cell reaching a fully charged voltage, the battery monitoring circuit (40) switches off the charging switch (31) to terminate the charging process.

This application is a continuation of Ser. No. 08/833,437 filed Apr. 7,1997, now U.S. Pat. No. 5,804,944.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to battery systems and, moreparticularly, to monitoring and protecting rechargeable batteries.

Lithium-ion batteries are preferred over other types of rechargeablebatteries such as nickel-cadmium batteries and nickel metal-hydridebatteries for portable electronics applications because of their lightweight and high energy density. However, lithium-ion batteries are verysensitive to overcharging, and safety is a major concern with their use.For example, a safety concern with lithium-ion batteries is thatmetallic lithium may plate onto one of the electrodes within the batterycell when it is overcharged. The plated lithium poses a fire hazardbecause of the flammable nature of metallic lithium. Another safetyconcern is the venting of noxious fumes when the temperature of thebattery cell becomes too high. Furthermore, in an over-dischargecondition, the voltage across a lithium-ion battery cell falls below anunder-voltage limit, resulting in a change in the chemical compositionof the electrolyte in the battery cell. Consequently, the life of thebattery cell may be significantly shortened. Therefore, it is importantto have a battery protection system that accurately monitors thelithium-ion batteries and ensures that they operate within their safeoperating areas.

Conventionally, charging a lithium-ion battery requires a dedicatedlithium battery charger. When the voltage of the lithium-ion batterybeing charged is significantly less than a fully charged voltage of thebattery, the dedicated lithium battery charger operates in a constantcurrent mode and charges the battery with a constant charging current.When the battery voltage is near the fully charged voltage of thebattery, the dedicated lithium battery charger switches to a constantvoltage mode. Under the constant voltage mode, the charging currentflowing in the battery decreases exponentially as the battery voltageapproaches the fully charged voltage, thereby preventing the batteryfrom becoming overcharged. The dedicated lithium battery chargerincludes a charge control circuit that determines when the dedicatedlithium battery charger switches from the constant current mode to theconstant voltage mode, how much charging current flows through thebattery during the constant voltage mode, and when the battery is fullycharged. In order to charge the battery to its maximum capacity andeffectively avoid overcharging, the charge control circuit is designedto be highly accurate. Typically, the voltage fluctuation of the chargecontrol circuit is less than one percent (%).

The highly accurate charge control circuit significantly increases thecost of the dedicated lithium battery charger and, therefore, increasesthe cost of using a lithium-ion battery. Further, because the chargingcurrent flowing in the battery decreases exponentially as the batteryvoltage approaches the fully charged voltage in the constant voltagemode, the charging process is time inefficient. For example, a lithiumbattery is usually charged up to 80% of its capacity during constantcurrent mode operation in a time interval ranging between one and twohours. Then, the dedicated lithium battery charger switches to theconstant voltage mode and takes at least three more hours to charge thelithium battery to its full capacity.

Accordingly, it would be advantageous to have a battery protectionsystem and a process for charging a battery. It is desirable for thesystem and the charging process to be cost efficient. It is alsodesirable for the charging process to be convenient and time efficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG 1 is a block diagram of a battery system in accordance with thepresent invention; and

FIG. 2 is a flow chart of a process for charging a battery in accordancewith the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Generally, the present invention provides a battery protection systemand a process for charging a battery. In accordance with the presentinvention, the battery protection system includes an accurate controlcircuit for controlling the battery charging process as well asprotecting the battery. Thus, the present invention eliminates the needfor a dedicated battery charger and, therefore, decreases the cost ofusing the battery. Further, the charging process of the presentinvention is time efficient compared with the prior art constantcurrent/constant voltage charging process.

FIG. 1 is a block diagram of a battery system 10 in accordance with thepresent invention. Battery system 10 includes a battery pack 15comprised of four serially coupled rechargeable lithium-ion batterycells 12, 14, 16, and 18. Therefore, battery system 10 is also referredto as a rechargeable battery system, and battery pack 15 is alsoreferred to as a rechargeable battery pack. Battery system 10 alsoincludes a battery protection system 20 which monitors battery pack 15and performs appropriate operations to ensure that battery pack 15operates in its safe operating area. Battery protection system 20 has apositive terminal 22 and a negative terminal 24, which serve as positiveand negative terminals, respectively, of battery system 10. Batteryprotection system 20 also has a positive battery electrode 26 and anegative battery electrode 28, which are connected to the positive andnegative terminals, respectively, of battery pack 15. Preferably,battery system 10 is an integral battery package, and battery pack 15and battery protection system 20 are assembled in the integral batterypackage.

Battery protection system 20 includes p-channel insulated gate fieldeffect transistors (FETs) 31 and 32, an inductor 34, a current sensingresistor 36, and a rectifier 38 comprised of a Zener diode 37 and aSchottky diode 39. An anode of Zener diode 37 and an anode of Schottkydiode 39 are connected together to form an anode electrode of rectifier38. A cathode of Zener diode 37 and a cathode of Schottky diode 39 areconnected together to form a cathode electrode of rectifier 38. FET 31has a source electrode connected to positive terminal 22 of batteryprotection system 20 and a drain electrode connected to a drainelectrode of FET 32. Inductor 34 has a first electrode connected to asource electrode of FET 32 and to the cathode electrode of rectifier 38and a second electrode connected to positive battery electrode 26 ofbattery protection system 20. Current sensing resistor 36 has a firstelectrode connected to negative battery electrode 28 of batteryprotection system 20 and a second electrode connected to the anodeelectrode of rectifier 38 and to negative terminal 24 of batteryprotection system 20.

Battery protection system 20 also includes a battery monitoring circuit40 having voltage sensing inputs 41, 42, 43, 44, and 45, current sensinginputs 46 and 47, and outputs 48 and 49. Voltage sensing input 41 isconnected to a positive electrode of battery cell 12. Voltage sensinginput 42 is connected to a negative electrode of battery cell 12 and toa positive electrode of battery cell 14. Voltage sensing input 43 isconnected to a negative electrode of battery cell 14 and to a positiveelectrode of battery cell 16. Voltage sensing input 44 is connected to anegative electrode of battery cell 16 and to a positive electrode ofbattery cell 18. Voltage sensing input 45 is connected to a negativeelectrode of battery cell 18. Current sensing inputs 46 and 47 areconnected to the first and second electrodes, respectively, of currentsensing resistor 36. Output 49 is connected to a gate electrode of FET32.

Battery protection system 20 further includes a comparator 52, ahysteresis comparator 54, and a FET driver 56. Comparator 52 has anon-inverting input and an inverting input connected to the first andsecond electrodes, respectively, of current sensing resistor 36.Hysteresis comparator 54 has a non-inverting input and an invertinginput connected to the first and second electrodes, respectively, ofcurrent sensing resistor 36. An output of comparator 52 is connected toan enabling terminal of hysteresis comparator 54. An output ofhysteresis comparator 54 is connected to a first input of FET driver 56.A second input of FET driver 56 is connected to output 48 of batterymonitoring circuit 40. An output of FET driver 56 is connected to a gateelectrode of FET 31.

Although FIG. 1 shows battery pack 15 having four battery cells, itshould be understood that this is not a limitation of the presentinvention. In accordance with the present invention, battery pack 15 mayinclude any number of battery cells, such as one, two, three, five, six,etc. Preferably, the number of voltage sensing inputs of batterymonitoring circuit 40 is such that the voltage of each battery cell inbattery pack 15 can be measured. It should also be understood thatbattery cells 12, 14, 16, and 18 are not limited to being lithium-ionbattery cells. They can be replaced by other types of battery cells suchas, for example, nickel-cadmium battery cells, nickel metal-hydridebattery cells, etc.

In battery protection system 20, FETs 31 and 32 serve as switches forinterrupting charging and discharging currents, respectively, flowingthrough battery pack 15 when the respective overcharge andover-discharge conditions develop. Because of their body diodes, FET 31only interrupts a charging current when switched-off and FET 32 onlyinterrupts a discharging current when switched-off. A charging currentis a current flowing through battery pack 15 from its positive terminalto its negative terminal. A discharging current is a current flowingthrough battery pack 15 from its negative terminal to its positiveterminal. It should be understood that FETs 31 and 32 are not limited tobeing insulated gate FETs. Any switching device having a controlelectrode and two current conducting electrodes can replace FET 31 orFET 32. As those skilled in the art are aware, when a FET serves as aswitch, the gate electrode of the FET functions as a control electrodeof the switch, and the source and drain electrodes of the FET functionas current conducting electrodes of the switch.

It should also be understood that battery protection system 20 is notlimited to employing high side switches, such as FETs 31 and 32, toprotect battery pack 15. In an alternative embodiment, batteryprotection system 20 includes two low side switches (not shown) coupledbetween current sensing resistor 36 and negative terminal 24 forinterrupting charging and discharging currents, respectively, flowingthrough battery pack 15 when the respective overcharge andover-discharge conditions develop. In another alternative embodiment,battery protection system 20 includes a high side switch, such as FET31, for interrupting the charging current and a low side switch (notshown) coupled between current sensing resistor 36 and negative terminal24 for interrupting the discharging current flowing through battery pack15 when the respective overcharge and over-discharge conditions develop.In yet another alternative embodiment, battery protection system 20includes a high side switch, such as FET 32, for interrupting thedischarging current and a low side switch (not shown) coupled betweencurrent sensing resistor 36 and negative terminal 24 for interruptingthe charging current flowing through battery pack 15 when the respectiveover-discharge and overcharge conditions develop. Preferably, a highside switch, such as FET 31 or FET 32, includes a p-channel insulatedgate FET, and a low side switch includes an n-channel insulated gateFET.

Resistor 36 functions as a current sensor that develops a voltage acrossits two electrodes when a current flowing through it. It can be replacedby other types of current sensing elements, e.g., a filament, etc. Inorder to minimize the power loss when battery system 10 supplies energyto a load (not shown) coupled across terminals 22 and 24, currentsensing resistor 36 preferably has a small resistance, e.g., less thanapproximately one ohm (Ω). In a preferred embodiment of the presentinvention, the resistance of current sensing resistor 36 isapproximately ten milli-ohms (mΩ).

Inductor 34, rectifier 38, comparator 52, and hysteresis comparator 54form charging control circuitry, which is referred to as a chargingcontroller or a charger interface circuit. The charging controllermonitors a battery charging current flowing in current sensing resistor36. When the battery charging current is above a first current value,the charging controller interrupts the battery charging current byswitching off FET 31, and induces a transient current flowing throughinductor 34, battery pack 15, current sensing resistor 36, and rectifier38. When the transient current is below a second current value, e.g.,substantially zero, that is less than the first current value, thecharging controller resumes the battery charging current by switching onFET 31.

In rectifier 38, Zener diode 37 prevents a large transient voltage dropacross FET 31 from occurring when a large discharging current flowing inbattery pack 15 is interrupted, and Schottky diode 39 provides a lowresistance conduction path when forward biased and, therefore, minimizesthe power consumption of rectifier 38. Preferably, Zener diode 37 has abreakdown voltage greater than a fully charged voltage of battery pack15. It should be understood that rectifier 38 can be replaced by othertypes of rectifying devices and circuit elements. For example, rectifier38 may include a single diode that has a breakdown voltage greater thanthe fully charged voltage of battery pack 15.

Comparator 52 turns on hysteresis comparator 54 and enables the chargingcontrol circuitry only when a charging current is flowing throughbattery pack 15. When there is a discharging current flowing throughbattery pack 15 or when battery pack 15 is idle, comparator 52 generatesa logic low voltage level at its output. The logic low voltage level istransmitted to the enabling terminal of hysteresis comparator 54 andturns off hysteresis comparator 54, thereby disabling the chargingcontrol circuitry and minimizing the current drain of battery system 10.It should be understood that, although preferred, comparator 52 is anoptional feature of the present invention.

Hysteresis comparator 54 switches FET 31 off via FET driver 56 when thecharging current flowing in battery pack 15 exceeds an upper limit.Hysteresis comparator 54 senses the charging current flowing in batterypack 15 by sensing a voltage drop across current sensing resistor 36. Asdiscussed hereinbefore, current sensing resistor 36 preferably has asmall resistance. Thus, hysteresis comparator 54 is preferably capableof switching FET 31 off via FET driver 56 in response to small inputsignals at the inputs of hysteresis comparator 54, e.g., input signalshaving a voltage on the order of ten milli-volts (mV). In addition,hysteresis comparator 54 may include a temperature compensation circuit(not shown), thereby providing a stable upper limit for the chargingcurrent over temperature variations in battery system 10.

FET driver 56 serves as a buffer that switches FET 31 on and off inresponse to the signals transmitted from battery monitoring circuit 40and hysteresis comparator 54 to the two inputs of FET driver 56.Preferably, FET driver 56 is able to switch FET 31 on and off at a highfrequency such as, for example, approximately 100 kilo-Hertz (kHz) orhigher. It should be understood that FET driver 56 is optional inbattery protection system 20. In an alternative embodiment of thepresent invention, the output of hysteresis comparator 54 and output 48of battery monitoring circuit 40 are directly coupled to the gateelectrode of FET 31. The frequency at which FET 31 is switched on andoff during the process of charging battery pack 15 is determined by theinductance of inductor 34. A larger inductance allows FET 31 to beswitched at a lower frequency, but increases the size, weight, and costof inductor 34. Preferably, the inductance of inductor 34 is in a rangebetween approximately one micro-Henry (μH) and approximately 100 μH. Anominal value for the inductance of inductor 34 is approximately 10 μH.

Battery monitoring circuit 40 periodically performs a safety monitoringoperation on battery pack 15. Through voltage sensing inputs 41, 42, 43,44, and 45, battery monitoring circuit 40 measures the voltage of eachof battery cells 12, 14, 16, and 18 in battery pack 15. Through currentsensing inputs 46 and 47, battery monitoring circuit 40 measures thecurrent flowing in battery pack 15 by measuring the voltage acrosscurrent sensing resistor 36. Based on these measurements, a controllogic circuit 50 in battery monitoring circuit 40 performs appropriateoperations to ensure that battery pack 15 operates in its safe operatingarea.

The safe operating area includes upper and lower limits for the voltageacross each of battery cells 12, 14, 16, and 18 in battery pack 15. Italso includes the upper limits for the charging and discharging currentsflowing through battery pack 15. If any of the safe operating limits areexceeded, control logic circuit 50 adjusts the corresponding parametersto be within their limits or terminates the condition which caused thecorresponding parameters to exceed their safe operating limits. Forexample, if an over-voltage condition is detected, control logic circuit50 switches FET 31 off and, if necessary, performs battery cellbalancing operations on battery pack 15. If an under-voltage conditionis detected, FET 32 is switched off, and battery protection system 20enters a hibernation state characterized by extremely low powerconsumption. Battery protection system 20 wakes up, i.e., leaves thehibernation state and returns to its normal operating state when acurrent flowing into positive terminal 22 is detected. If anover-current condition is detected, control logic circuit 50 switchesoff either FET 31 or FET 32 to terminate the over-current condition. Itshould be noted that FET 31 is switched off if the over-current isflowing in battery pack 15 in a direction from positive batteryelectrode 26 to negative battery electrode 28, and FET 32 is switchedoff if the over-current is flowing in battery pack 15 in a directionfrom negative battery electrode 28 to positive battery electrode 26. Inorder to ensure safe operation and achieve maximum energy efficiency ofbattery pack 15, control logic circuit 50 in battery monitoring circuit40 is designed to be highly accurate. Typically, the voltage fluctuationof control logic circuit 50 is preferably less than approximately onepercent (%).

It should be understood that battery monitoring circuit 40 is notlimited to monitoring the voltage of each battery cell and the currentflowing in battery pack 15. In an alternative embodiment of the presentinvention, battery monitoring circuit 40 also monitors an ambienttemperature of battery pack 15. In another alternative embodiment of thepresent invention, battery monitoring circuit 40 includes a battery celldetection circuit (not shown) that detects the number of battery cellsin battery pack 15. The operation of a battery control circuit such asbattery monitoring circuit 40 is described in co-pending U.S. patentapplication Ser. No. 08/398,255, attorney's docket No. SC09078C,entitled "CIRCUIT AND METHOD FOR BATTERY CHARGE CONTROL", by Troy L.Stockstad et al. and assigned to Motorola, Inc. U.S. patent applicationSer. No. 08/398,255 is hereby incorporated herein by reference.

FIG. 2 is a flow chart 60 of a process for charging a battery inaccordance with the present invention. By way of example, the battery isshown in FIG. 1 as battery pack 15 coupled to battery protection system20.

To charge battery pack 15 in battery system 10 of FIG. 1, the positiveand negative terminals of a voltage source (not shown) are coupled topositive terminal 22 and negative terminal 24, respectively, of batterysystem 10. FET 31 is switched on by a logic low voltage level at itsgate electrode. A charging current is generated (reference numeral 61 inFIG. 2) and flows from the positive terminal of the voltage source (notshown) through conductive FET 31, FET 32, inductor 34, battery pack 15,and current sensing resistor 36, to the negative terminal of the voltagesource (not shown). Rectifier 38 is reverse biased and nonconductive.Because of inductor 34, the charging current increases gradually fromzero. Further, a portion of the electrical energy in the chargingcurrent is converted to electromagnetic energy. In other words,electromagnetic energy is generated using the charging current(reference numeral 62 in FIG. 2) and stored in inductor 34.

The charging current develops a voltage difference across currentsensing resistor 36. Comparator 52 senses the voltage difference acrosscurrent sensing resistor 36. With its non-inverting input at a highervoltage level than its inverting input, comparator 52 generates a logichigh voltage level at its output. The logic high voltage level istransmitted to the enabling terminal of hysteresis comparator 54.Hysteresis comparator 54 is enabled and senses the voltage differenceacross its two inputs, thereby sensing the charging current flowing inbattery pack 15 and current sensing resistor 36 (reference numeral 63 inFIG. 2).

The voltage difference across the two inputs of hysteresis comparator 54is proportional to the current flowing in battery pack 15. When thecharging current flowing through battery pack 15 is smaller than apredetermined current value, e.g., approximately 3 amperes (A), thevoltage difference across the two inputs of hysteresis comparator 54 issmaller than a first threshold voltage value of hysteresis comparator54, e.g., approximately 30 mV. Hysteresis comparator 54 generates alogic low voltage level at its output. The logic low voltage level istransmitted to the gate electrode of FET 31 via FET driver 56. FET 31remains conductive, and the charging current continues to flow throughbattery pack 15. When the charging current rises beyond thepredetermined current value, e.g., approximately 3 A, the voltage levelat the non-inverting input of hysteresis comparator 54 is higher thanthat at the inverting input of hysteresis comparator 54 and the voltagedifference is greater than the first threshold voltage value, e.g.,approximately 30 mV. Hysteresis comparator 54 generates a logic highvoltage level at its output. FET driver 56 transmits the logic highvoltage level to the gate electrode of FET 31, which switches off tointerrupt the charging current flowing from the voltage source (notshown) to battery pack 15 (reference numeral 64 in FIG. 2).

In response to FET 31 being switched off and the charging current beinginterrupted, the electromagnetic energy stored in inductor 34 isreleased to generate a transient current flowing through battery pack15, current sensing resistor 36, and forward biased rectifier 38(reference numeral 65 in FIG. 2). Hysteresis comparator 54 continues tosense the voltage difference across its two inputs and, therefore,senses the transient current flowing in battery pack 15 (referencenumeral 66 in FIG. 2). When the transient current decreases to anotherpredetermined current value, e.g., a current value which issubstantially equal to zero, the voltage difference across the twoinputs of hysteresis comparator 54 falls to a corresponding voltagevalue, e.g., a voltage value substantially equal to zero, which is lessthan a second threshold voltage of hysteresis comparator 54. The voltagelevel at the output of hysteresis comparator 54 switches back to thelogic low voltage level. The logic low voltage level is transmitted tothe gate electrode of FET 31 via FET driver 56, thereby switching on FET31 and regenerating the charging current flowing from the voltage source(not shown) to battery pack 15 via conductive FET 31 (reference numeral67 in FIG. 2). Then, inductor 34 repeats the step of generatingelectromagnetic energy using the charging current (reference numeral 62in FIG. 2), and hysteresis comparator 54 repeats the step of sensing thecharging current flowing in battery pack 15 (reference numeral 63 inFIG. 2).

As described hereinbefore, the current flowing in battery pack 15 duringthe charging process is a pulsed current modulated by FET 31, inductor34, and hysteresis comparator 54. The frequency at which the chargingcurrent is interrupted and regenerated depends on the inductance ofinductor 34, a charging voltage supplied by the voltage source (notshown) coupled to positive terminal 22 and negative terminal 24, thevoltage of battery pack 15, and the threshold voltages at whichhysteresis comparator 54 switches its output voltage level. Generally, asmall inductance and/or a large voltage difference between the chargingvoltage and the voltage of battery pack 15 will result in a highfrequency at which the charging current is interrupted and regenerated.Typically, the frequency is between approximately 50 kHz andapproximately 200 kHz. A nominal frequency is approximately 100 kHz.

During the charging process, control logic circuit 50 in batterymonitoring circuit 40 periodically generates a logic high voltage levelat output 48. The logic high voltage level at output 48 is transmittedto the gate electrode of FET 31 via FET driver 56 and switches FET 31off, thereby interrupting the charging current flowing in battery pack15. Preferably, FET 31 remains nonconductive for a time interval that issufficiently long for the transient current flowing in battery pack 15to decrease to substantially zero. Battery monitoring circuit 40 thensenses the voltage across each of battery cells 12, 14, 16, and 18 inbattery pack 15 (reference numeral 68 in FIG. 2). Because the currentflowing in battery pack 15 is substantially zero, the voltage of each ofbattery cells 12, 14, 16, and 18 sensed by battery monitoring circuit 40is substantially equal to an open circuit voltage of the respectivebattery cell. The sensed voltage of each of battery cells 12, 14, 16,and 18 is compared with a reference voltage in control logic circuit 50(reference numeral 69 in FIG. 2). Preferably, the reference voltage isequal to a fully charged voltage of each of battery cells 12, 14, 16,and 18.

If the sensed voltage of each of battery cells 12, 14, 16, and 18 islower than the reference voltage, control logic circuit 50 generates alogic low voltage level at output 48. The logic low voltage level istransmitted to the gate electrode of FET 31 via FET driver 56. FET 31 isswitched on, and the steps of generating the charging current,generating the electromagnetic energy, sensing the charging current,interrupting the charging current, generating the transient current,sensing the transient current, regenerating the charging current, andsensing the voltage across each of battery cells 12, 14, 16, and 18(reference numerals 61, 62, 63, 64, 65, 66, 67, and 68 respectively, inFIG. 2) are repeated.

If the sensed voltage of a battery cell in battery pack 15 issubstantially equal to or higher than the reference voltage, the voltagelevel at output 48 of battery monitoring circuit 40 remains at the logichigh voltage level. FET 31 is latched off and the charging process isterminated (reference numeral 70 in FIG. 2). If necessary, batterymonitoring circuit 40 performs the battery cell balancing operation onbattery pack 15 by discharging the battery cell that has the highestvoltage. After the battery cell balancing operation, battery monitoringcircuit 40 may restart the charging process by switching on FET 31 ifthe voltage of each of battery cells 12, 14, 16, and 18 in battery pack15 is lower than the reference voltage in control logic circuit 50.

The frequency at which battery monitoring circuit 40 switches FET 31 offand senses the voltage across each of battery cells 12, 14, 16, and 18in battery pack 15 during the charging process is determined by a timer(not shown) in control logic circuit 50. The timer also determines howlong FET 31 remains nonconductive each time FET 31 is switched off bybattery monitoring circuit 40. By way of example, battery monitoringcircuit 40 switches FET 31 off once in a time period of approximatelyone second (s) during the charging process. Further, FET 31 remainsnonconductive for a time interval of approximately twenty milli-seconds(ms) each time it is switched off by battery monitoring circuit 40.However, it should be understood that the frequency and time interval ofFET 31 being switched off by battery monitoring circuit 40 during thecharging process are not limited to these values. In alternativeembodiments of the present invention, FET 31 may be switched off bybattery monitoring circuit 40 once for a time interval of 10 ms, 15 ms,or 25 ms in a time period of 0.5 s, 1.5 s, or 2 s during the process ofcharging battery system 10. By switching off FET 31 at a higherfrequency and for a longer time interval, the voltage of each of batterycells 12, 14, 16, and 18 can be monitored more closely and accurately,ensuring battery pack 15 operates in the safe operating area. However,the charging process would be less time efficient.

Accordingly, FET 31, inductor 34, current sensing resistor 36, rectifier38, battery monitoring circuit 40, comparator 52, hysteresis comparator54, and FET driver 56 cooperate to control the charging current flowingin battery pack 15 during the process of charging battery pack 15 inbattery system 10. In other words, control logic circuit 50 in batterymonitoring circuit 40 serves to control the charging process as well asto protect battery pack 15. Thus, the need for a dedicated lithiumbattery charger that includes a highly accurate charge control circuitis eliminated. Battery system 10 can be coupled to an unregulatedvoltage source to charge battery pack 15. In other words, the redundancyof having two accurate control circuits, one in the battery protectionsystem and the other in the dedicated lithium battery charger, asrequired in the prior art charging process is eliminated. The cost ofusing a lithium-ion battery pack coupled to battery protection system 20and packaged in battery system 10 is significantly reduced compared withthe cost of using a lithium-ion battery pack coupled to a prior artbattery protection system. Further, the current flowing in battery pack15 during the charging process is a pulsed current. The average chargingcurrent flowing in battery pack 15 is larger than the average chargingcurrent in the prior art constant current/constant voltage chargingprocess. Therefore, the process of charging battery system 10 inaccordance with the present invention is more time efficient than theprior art constant current/constant voltage charging process.

By now it should be appreciated that a battery protection system and aprocess for charging a battery have been provided. In accordance withthe present invention, an accurate control circuit in the batteryprotection system controls the battery charging process as well asprotects the battery pack, thereby eliminating the need for a dedicatedbattery charger. The battery can be charged using an unregulated voltagesource, which is inexpensive, widely available, and easy to use. Thus,charging the battery in accordance with the present invention isconvenient and cost efficient. Further, the charging process of thepresent invention is more time efficient than the prior art constantcurrent/constant voltage charging process.

We claim:
 1. A process for charging a battery, comprising the stepsof:generating a first current flowing through the battery; interruptingthe first current in response to the firstcurrent being greater than afirst current value; inducing a second current flowing through thebattery in response to the first current being interrupted; regeneratingthe first current in response to the second current being less than asecond current value, the second current value being smaller than thefirst current value; sensing a voltage across the battery; and repeatingthe steps of generating the first current, interrupting the firstcurrent, inducing the second current, and regenerating the first currentin response to the voltage being less than a voltage value.
 2. Theprocess as claimed in claim 1, wherein:the step of generating a firstcurrent further includes the step of generating an electromagneticenergy from a portion of an electrical energy in the first current; andthe step of inducing a second current includes inducing the secondcurrent by releasing the electromagnetic energy.
 3. The process asclaimed in claim 1, wherein the step of inducing a second currentfurther includes establishing a conduction path from a negative terminalof the battery to a positive terminal of the battery via a rectifier. 4.The process as claimed in claim 1, wherein the step of sensing a voltageacross the battery includes sensing a voltage across a battery cell of aplurality of serially coupled battery cells in the battery.
 5. Theprocess as claimed in claim 1, wherein the step of sensing a voltageacross the battery further includes the steps of:interrupting the firstcurrent; and sensing an open circuit voltage of across a battery cell ofa plurality of serially coupled battery cells in the battery when thesecond current is substantially zero.
 6. The process as claimed in claim1, wherein the step of sensing a voltage across the battery includesperiodically sensing the voltage across the battery.